A network data storage device is a device or system used for centrally storing or retrieving large amounts of data for multiple network nodes. While there are various types of conventional network data storage devices, one exemplary type is a sequential media network data storage device. A sequential media network data storage device is implemented with one or more drives that are operable with at least one removable media element, such as an optical or magnetic media. Optical media is typically formed as a disk whereas magnetic media is typically formed as a tape on a spool or in a cartridge.
In the enterprise arena, sequential media network storage devices implemented with libraries of magnetic media are widely used. Their degree of market penetration is attributable to their large capacities and low cost per unit of storage. However, sequential media network data storage devices do have drawbacks, namely access time delays, susceptibility to data loss and underutilization.
There are several time delays that may occur when accessing a magnetic media. Typically, a drive is able to load only one magnetic tape at a time. Accordingly, there is time delay associated with retrieving the magnetic tape from the library and loading the magnetic tape in the drive. Further, if another magnetic tape is loaded in the drive, that magnetic tape must be rewound and unloaded. Thus, there is an additional time delay experienced that is equal to the amount of time it takes to rewind and unload magnetic tape from the drive. Still further, once magnetic tape is loaded in a drive, it may have to be located to the appropriate position, thereby adding to the time delay. In it not uncommon for the aggregate of the above time delays to be in excess of five minutes. Accordingly, sequential media network storage devices implemented with libraries of magnetic media experience significant time delays when accessing magnetic media. Conventional implementations fail to address the problem of time delay and, as describe below, actually perpetuate it.
Magnetic tape is highly susceptible to data loss due to positional changes and the overwriting of data. In a typical network scenario, there are multiple entities that could access and write data to a drive. Accordingly, without a mechanism in place to control drive access, data may be overwritten. The solution in the conventional art, as described in detail below, is to have a centralized entity manage access to a drive by coordinating an exclusive reservation of the drive. While a network entity posses an exclusive reservation to a drive, other network entities are prevented from accessing the drive. Thereby, data loss is mitigated. However the conventional solution is only partly effective in that the drive is venerable to data loss at times between reservations. Further, the conventional art perpetuates access time delay in that the conventional reservation mechanism requires the loading and unloading of the magnetic tape in order to transfer hosts. Still further, the conventional solution leads to an underutilization of the magnetic media in a media library.
FIG. 2 is a flowchart of an exemplary conventional method for a host to host transfer of media. In step 201, a controlling server manages client storage tasks. The management includes making the selection of the host/drive/media combination for the next storage task based on information obtained from a master server. Herein, the controlling server selects a drive/media combination that is coupled with a non-selected host. In other words, the controlling server has selected a media to use with the selected host that is exclusively reserved by a non-selected host.
To initiate the transfer, the controlling server, in step 203, issues a release command instructing the non-selected host to physically rewind and unload the media, and release the exclusive reservation. In step 205, the non-selected host responds by controlling the selected drive to physically rewind and unload the media. After which, in step 207, the non-selected host releases the exclusive reservation of the selected drive. The host then reports the release of the exclusive reservation to the controlling host in step 209. At this time, both the drive and media are vulnerable to acquisition by another host acting outside the control of the controlling server. In step 211, the controlling server issues a reserve command instructing the selected host to establish a reservation with the selected drive and to load the selected media.
In step 213, it is determined if the selected host successfully establishes the reservation. Such determination is typically made by the selected host and reported to the controlling server. However, the controlling server may make this determination itself or via another intermediary. If the exclusive reservation was not successful, the method returns to step 201. If the exclusive reservation is established, the selected host loads the selected media in the selected drive in step 215. The selected host then performs the client storage task using the selected media in the selected drive. The selected host reports the completion of the storage task to the controlling server in step 219. After which, control returns to step 201.
Accordingly, the above described conventional method for host to host transfer of media requires that even if the media is to be used in the same drive it is loaded in, it has to be physically unloaded and reloaded which results in time delay and additional wear and tear on the loading/unloading mechanism. Further, the releasing and subsequent establishment of the reservations by the hosts leaves the drive and media vulnerable to acquisition or data loss. In addition, when a reservation error occurs there is no mechanism in place by which to recover the drive.
In view of the foregoing, it would be desirable to provide techniques for host to host transfer of sequential media and the protection of sequential media during host to host transfer which overcomes the above-described inadequacies and shortcomings.